`xenium` fails on imported segmentation

[dcjones]

The xenium function assumes there are two masks corresponding to nuclear and cell boundaries, which is true of xenium segmentation, but this not necessarily true when 3rd party segmentation is imported using the xeniumranger import-segmentation tool.

Issue is here:

spatialdata-io/src/spatialdata_io/readers/xenium.py

Line 232 in 58b2f5d

mask_index=1,

If not detecting this case automatically, there should be at least a function argument to point it to the correct mask index.

(See dcjones/proseg#75)